Low-dose turbidity control using cationic polysaccharide bioflocculants: performance, dosage tolerance and molecular interpretation

Abstract Bioflocculants are being developed as sustainable alternatives to inorganic coagulants for turbidity control, but how polysaccharide backbones affect the performance of cationically modified bioflocculants remains insufficiently understood. Here, modified starch (M-Starch) and modified chitosan (M-Chitosan) carrying quaternary ammonium groups were compared in kaolin suspensions under high- and low-turbidity conditions. In the high turbidity system, M-Starch achieved over 95% turbidity removal at 0.3–0.5 mg/L and showed a wider effective dosage range than M-Chitosan. Under low turbidity conditions, M-Starch achieved approximately 55% removal, compared with about 37% for M-Chitosan and less than 10% for Al₂(SO₄)₃. Zeta potential results showed that the optimum performance of M-Starch did not coincide with complete charge neutralization. Charge equivalent comparison and graft-chain estimates further indicated that the different dose responses could not be explained solely by the amount of charge introduced or by graft-chain parameters. Electronic structure calculations and molecular dynamics simulations further indicated that M-Starch tended to form a compact conformation with heterogeneous charge distribution, whereas M-Chitosan showed a more extended configuration with a more uniform cationic surface. The results indicate that backbone-associated structural features may modulate the dose response of cationic bioflocculants by influencing charge accessibility, molecular conformation and hydration behaviour.